Aerosol containers are used for dispensing many different fluent products, from products of very low viscosity to products of very high viscosity. One type of aerosol container is a piston container on a barrier pack, comprising a container having a piston dividing it between a chamber on the outlet nozzle side of the container in which chamber the fluent product to be dispensed is located and a chamber for containing a gaseous propellant on the other side of the piston. The propellant continuously urges the piston toward the nozzle for dispensing the fluent material from the aerosol container when the valve in the nozzle is opened.
Depending upon various factors, the pressure exerted by the propellant on the fluent material can vary, between an aerosol container that is fully charged and one that is nearly fully discharged, and between a high pressure container and a low pressure container, with a container pressure anywhere between 6 psig. through about 150 psig. The material of which the container is comprised can vary from soft cardboard, through thin plastic, thick plastic, thin metal, up to heavy weight metal, with the choice of material being dependent in part upon the maximum pressure in the fully charged aerosol container.
The walls of an aerosol container are often slightly dented, nicked or otherwise deformed during manufacture, shipping, storage, delivery and/or use. A higher pressure container will at least partially restore itself to its original shape due to the internal pressure therein. But, a low pressure container and/or one made of inflexible or non-resilient material may not restore itself and may retain its deformed condition. The walls of a soft walled container can be easily restored to their original shape. But, the walls of a harder walled container usually retain any dents or deformations.
It is desirable for the piston in any aerosol container to maintain a continuous annular seal with the interior wall of the container as it moves through the container toward the nozzle. When the container wall has no significant dents or deformations, maintenance of this seal is easy. However, when the container wall does have dents and deformations, as the piston passes by these, the seal of the piston may be broken. With a soft walled container, it is beneficial if the annular periphery of the piston, which engages the internal side walls of the container, is sufficiently rigid to restore the container wall to its original undeformed shape as the piston passes by. This helps maintain the seal. In the case of hard walled containers whose walls are deformed or in the case of those deformations in a soft walled container which cannot be easily straightened out as the piston moves by, it is desirable for the piston to have sufficient flexibility to itself deform to conform in shape to the deformation in the container as it passes the deformation. At the same time, the piston should have sufficient elasticity and resilience to restore itself to the interior profile of the container walls once the piston has passed the deformation, thereby to maintain the continuous seal.
Most aerosol container barrier pistons are loaded through the bottom of the container, which is the end opposite the outlet nozzle. However, certain containers, and in particular soft walled containers and also extruded cans, are initially formed with their bottom in place, and the barrier piston is installed through the top of the closed bottom container. It is desirable, therefore, for the piston to be capable of being received through the open top of the container. Furthermore, the rim portion of the open top of an aerosol container is often slightly narrower in diameter, as compared with the diameter of the remainder of the container interior wall. The piston should be so designed that it can be inserted through the narrowed top rim portion of the container and yet still maintain a seal with the entire length of the interior wall of the container.
Furthermore, as the barrier piston moves through the aerosol container, during expulsion of fluent material, the piston sometimes cocks and does not always maintain an orientation at which its axis is aligned with the axis of the container. Pistons typically have an anti-cocking means which prevents them from tilting or cocking through too great an angle. But, a certain minimum degree of cocking is almost unavoidable, and a piston should be adapted to maintain a secure seal, even though the piston has cocked slightly.
Finally, as a piston moves up through the aerosol container toward the outlet, all of the contents of the container above the piston should be forced out of the valve in the outlet nozzle. None of the contents should be able to slip down past the side of the piston and none should adhere to the container wall as the piston goes by. However, a very effective seal for preventing by-pass of the contents of the container may undesirably hinder movement of the piston through the container. Thus, it is desirable to provide some means on the piston for minimizing or eliminating the passage of any of the contents of the container past the piston while still permitting the piston to move through the container.